1. Field of the Invention
This invention is generally related to acoustic data analysis, and more particularly to automated estimation of fluid slowness to facilitate acoustic logging and analysis
2. Background of the Invention
Formations are characterized in terms of slowness values. For example, a formation may be characterized as being slow if the shear slowness, i.e., inverse of velocity, of the formation is greater than the mud slowness. If the shear slowness of the formation is less than the mud slowness then the formation may be characterized as being fast. As described in Cheng, C. H., and Toksoz, M. N., 1981, Elastic wave propagation in a fluid filled borehole and synthetic acoustic logs, Geophysics, 46, p. 1042, in fast formations it is known to utilize a monopole source, where refracted compressional arrival time, refracted shear arrival time, and a Stoneley wave which is guided by the fluid-rock interface are excited by the monopole source. These various arrivals are usually used to estimate, respectively, compressional, shear and Stoneley slowness of the formation. As described in Paillet, F. L. and Chang, C. H., 1991, Acoustic waves in borehole: CRC Press Inc, ISBN 0-8493-8890-2, Boca Raton, Ann Arbor, Boston, London, it is also possible to use other modes such as leaky modes to get an estimate of compressional slowness in a slow formation. However, in slow formations it is not possible to measure formation shear slowness from headwaves because shear waves do not exist in slow formations. It is known to use a dipole transmitter that excites dipole flexural waves in the borehole in order to overcome this limitation. Like other borehole modes, the dipole mode is dispersive (See Sinha, B. K. and Zeroug, S., 1997, Geophysical prospecting using sonics and ultrasonics: Wiley Encyclopedia of Electrical and Electronic Engineers, John G. Webster, Editor, John Wiley and Sons, Inc.). However, it is possible to estimate formation shear slowness by extracting the dipole slowness at low frequencies as described by Kimball, C. V, and Marzetta, T. L., 1987, Semblance processing of borehole acousticg data, Geophysics, 49, 530-544.
One factor that affects acoustic wave propagation measurements in a fluid filled borehole is the fluid slowness, e.g., mud slowness, where mud is disposed between the tool and the borehole wall. There is no practical technique for measurement of the mud slowness in a well at sonic frequencies. Various indirect and direct evaluation techniques are known. However, these techniques have some drawbacks.
Indirect evaluation of mud slowness can be based on examination of mud samples at the surface or data from the manufacturer of the mud components. However, these techniques tend to be inaccurate because mud slowness is a function of conditions which can differ significantly between the surface and locations of interest within the well, e.g., pressure, temperature, presence/absence of gas, etc. Empirical equations have been developed that describe some common mud types, but errors can still occur if incorrect assumptions about conditions are used, or if the uncertainties of some parameters are too large.
Direct evaluation of mud slowness can be based on the dispersive characteristics of some modes using a Prony-based method as described by Lang, S. W., Kurkjian, A. L., McClellan, J. H., Morris, C. F., and Parks, T. W., 1987, Estimating slowness dispersion array from arrays of sonic waveforms: Geophysics, 52 (4), 530-544. The technique involves transforming an array of time waveforms into a frequency slowness domain to enable evaluation of the characteristics of the various dispersive and non-dispersive modes present in the recorded data, as described by Plona, T., Sinha, S., Kane, M., Bose, S., Wang, C., Pabon, J., Zeroug, S., 2004, Identifying formation response using sonic dispersion curves, 74th Annual International Meeting of the Society of Exploration Geophysicists (SEG), Denver, Expanded Abstracts. Various options are available for performing this analysis, depending on the formation type and modes considered. One option includes adjusting the mud slowness in the modeling parameters to match the Stoneley dispersion curve model to the dispersion curve computed from the data. Another option is based on the fact that mud slowness is asymptotically approached by both the Stoneley and flexural data. The asymptote of the Stoneley dispersion curve at high frequency must be slower or equal to the mud value, while the value of the shear asymptotes must be faster than the mud (unless the formation is damaged). The dipole-flexural curve converges to the Scholte slowness, which is dependent on both mud slowness and the formation properties close to the borehole wall. Another option is based on the Leaky P mode. However, this option is only valid when a leaky compressional is present in the data, i.e., in a slow formation. The leaky modes can be considered as multiple reflected and constructively interfering waves propagating in the borehole, as described by Tichelaar, B. W. and Luik K. W., 1995, Sonic logging of compressional-wave velocities in a very slow formation, Geophysics, 60, 1627-1633; and Valero, H. P., Peng, L., Yamamoto, M., Plona, T., Murray, D., Yamamoto, H., 2004, Processing of monopole compressional in slow formation, 74th Annual International Meeting of the Society of Exploration Geophysicists (SEG), Denver, Expanded Abstracts. Unlike the refracted P head wave, leaky modes are dispersive, i.e., starting at the compressional velocity at low frequency and tending to the mud velocity as frequency increases. Further, there exists a cutoff frequency below which they are not excited. Although such dispersion analysis may be used to estimate mud slowness, the technique requires time-consuming analysis of various frames by skilled personnel. Further, none of the techniques is suitable for all formations.